The present invention relates to a torque sensor. More particularly, the present invention pertains to magnetostrictive torque sensors that are attached, for example, to a drive shaft or a steering shaft of vehicles to detect the magnitude of torque applied to the shaft and the rotational direction and rotational amount of the shaft.
FIG. 11 is a cross-sectional view illustrating a prior art magnetostrictive torque sensor 81 provided on a shaft 80. The sensor 81 includes a sleeve 82 fitted about the shaft 80. A magnetostrictive layer 83 is formed on the circumference of the sleeve 82. The layer 83 is located at the axial center portion of the sleeve 82. The layer 83 is divided into two magnetostrictive segments 83A, 83B at the axial center of the sleeve 82 by an imaginary plane perpendicular to the axis of the shaft 80. Grooves are formed in the outer surfaces of the layer 83. Half of the grooves are formed in the segment 83A and the other half are formed in the segment 83B. The grooves on one of the segments 83A, 83B are inclined by forty-five degrees relative to the axis of the sleeve 82, whereas the grooves on the other segment are inclined by minus forty five degrees. The grooves on the segment 83A and the grooves on the segment 83B do not intersect with each other.
Bearings 84 are press fitted about the sleeve 82. The bearings 84 support a cylindrical stator 85 to be rotatable relative to the sleeve 82. Exciting coils 86A, 86B and detecting coils 87A, 87B are located in the inner wall of the stator 85. The coils 86A, 87A face the segment 83A and the coils 86B, 87B face the segment 83B.
The segments 83A, 83B are strained in accordance with the magnitude and direction of torque acting on the shaft 80. The strain of the segments 83A, 83B changes the permeability of the segments 83A, 83B. Changes in the permeability cause voltages induced by the detecting coils 87A, 87B to differ from each other. The value of the torque applied to the shaft 80 is represented by the magnitude of the voltage difference. The direction of the torque is represented by the sign of the subtraction result of the voltages. The magnetostrictive sensor detects the torque based on the difference between the output voltages of the coils 87A, 87B.
With no torque applied on the shaft 80, the segments 83A and 83B have the same permeability. However, if the segments 83A, 83B are axially displaced relative to the corresponding coil pairs 86A, 87A and 86B, 87B, magnetic fluxes through the detecting coils 87A, 87B are changed. As a result, the voltage of the coils 87A, 87B become different even if there is no torque applied to the shaft 80. This results in an erroneous detection of torque. In other words, the detection accuracy of the torque sensor 81 is degraded.
Accordingly, the torque sensor 81 has annular collars 88 for preventing the segments 83A, 83B from being displaced relative to the coil pairs 86A, 87A and 86B, 87B. The collar 88 are formed on the circumference of the sleeve 82 and prevent the bearings 84, which are press fitted about the sleeve 82, from moving axially. The bearings 84 hold the stator 85 thereby restricting the axial movement of the stator 85. As a result, the positions of the coil pairs 86A, 87A and 86B, 87B are fixed relative to the segments 83A, 83B.
However, each element in the torque sensor 81 has a dimensional error that is created during manufacture process. Thus, the diameter of the sleeve 82 may be slightly smaller than the inner diameter of the bearings 84. In this case, the bearings 84 are not firmly fitted about the sleeve 82. This results in the bearings 84 being slightly inclined relative to the sleeve 82. In other cases, the bearings 84 separate from the collars 88 during use of the sensor 81. As a result, the coil pairs 86A, 87A and 86B, 87B are axially displaced from the predetermined positions in relation with the segments 83A, 83B. This degrades the detection accuracy of the sensor 81.
In order to cope with the above drawbacks, a construction including snap rings fitted about the sleeve 82 is proposed. Specifically, a groove is formed in the sleeve 82 at a position adjacent to and axially outside of each bearing 84. A snap ring is fitted to each groove. Each snap ring and the corresponding bearing 84 fix the position of the corresponding bearing 84. However, the width of the grooves and thickness of the snap ring include dimensional errors. The errors may result in the bearings 84 separating from the collars 88. As a result, the position of the stator 85 is displaced relative to the sleeve 82.